This application relates generally to systems and methods for measuring fluid flow through a pipe, and more particularly to systems and methods for providing highly accurate real-time mass flow measurements.
In the context of aircraft, an accurate mass flow of fuel must be determined in order to determine how much fuel remains in reserve throughout flight. Torque/angle mass flow meters have been used on aircraft since the 1950's, but have limited turn down ratios (flow ranges) and exhibit very poor accuracy except over certain temperature and flow bands. Robust and highly accurate volumetric turbine flow meters have been available, but these are not true mass flow meters. These military standard turbine flow meters, having excellent turn down ratios (min/max flow) and dual bearing systems are made by companies such as Quantum Dynamics of Woodland Hills, Calif. In addition to military aircraft, Quantum Dynamics' flow meters have been used on lunar excursion modules and have some limited use on certain commercial aircraft. High accuracy coriolis meters have also been implemented for measuring flow rates in ground-based applications. However these meters are expensive, complex, and have other shortcomings such as sensitivity to vibration and g-forces, making them unsuitable for aircraft use.
While turbine flow meters have very good volumetric flow capabilities, they rely on nominal fluid density or predefined estimates tuned for certain operating conditions. This causes significant deviation from the actual fluid mass flow if there is substantial variation in the fluid characteristics. As there is a level of acceptable variation in the composition of many process fluids such as Jet/A1 fuel, the density of the fluid can vary, affecting mass flow measurements. Since Jet/A1 varies from one region to another, depending on where the crude is sourced and/or where it is refined, Jet/A1 from multiple sources with multiple densities can be mixed in processing, transport or in aircraft fuel tanks both prior to and during flight. Therefore the actual density of the fuel entering the engine at any given moment cannot be predicted, and using a nominal value of Jet/A1 density will nearly always yield substantial mass flow error even using otherwise highly accurate volumetric flow meters.